Immense heavy oil reservoirs exist in many parts of the world. Heavy oil endowments are hundreds of billions of barrels in some nations. However, much of the heavy oil in place can be uneconomical to produce depending on factors such as conventional oil price, distance to markets, and refining complexity. One of the primary determinants of oil value is its viscosity.
Heavy oil reservoirs may contain several grades of oil. For example, a reservoir might be composed of stacked porous beds separated by impermeable layers. Each bed can contain oil with properties different from the oils in beds above and below. It is believed that fluid properties can vary even within a single bed, so that, for example, viscosity increases from the top to the bottom of the bed.
Producers seeking to select their best prospects generally want to locate the beds containing oil with the lowest viscosity, among other desirable properties. Borehole logging tools generally are the most accurate and cost-effective means of determining properties of fluids found in subsurface geological formations. One method that has been found suitable for this purpose is formation sampling. In a form of this technique a borehole logging tool (e.g. the Schlumberger “MDT™” tool) extracts fluid from the formation, stores it in a sample bottle, and conveys the bottle to the surface. The sample is then typically transferred to another bottle for shipment to a fluids analysis laboratory. Fluids laboratories are capable of determining a variety of physical and chemical properties, including viscosity.
Despite the success of downhole fluid sampling, a number of factors limit the effectiveness of this technique, as described below. (1) Only a relatively small number of oil samples can be obtained in each descent of the tool into the borehole, typically only six or twelve samples. (2) It is time consuming to obtain a good sample, typically an hour or more. This is a disadvantage due to the very high cost of operating drilling rigs. (3) Transporting the samples to a laboratory and having them characterized there can take weeks, which can impede the production schedule. (4) Sample integrity can be jeopardized by temperature and/or pressure changes associated with uphole handling, and by the transfer of samples to and from transportation cylinders. (5) It is not practical to obtain samples of oils with viscosity greater than several thousand centipoise [see, for example, J. A. Cañas, S. Low, N. Adur and V. Teixeira, “Viscous Oil Dynamics Evaluation For Better Fluid Sampling”, SPE/PS-CIM/CHOA 97767, SPE International Thermal Operations and Heavy Oil Symposium, Calgary, Alberta, Canada, 1-3 Nov. 2005].
Some of the listed limitations can be mitigated by making measurements of fluids such as oils within the fluid sampling tool itself. One such measurement is nuclear magnetic resonance (NMR) [see, for example, U.S. Pat. Nos. 3,528,000; 6,107,796; 6,111,408; 6,111,409; 6,346,813; 6,825,657; 6,841,996; and U.S. Pat. No. 6,891,369]. In some approaches, fluids in a flowline of fluid sampling tool are subjected to NMR measurements while downhole. Techniques of the present invention apply, inter alia, to NMR measurements in fluid sampling tools, as well as to various other types of measurements, as will be described.
Nuclear magnetic resonance logging of earth formations surrounding a borehole is another way of estimating the viscosity of heavy oils. Although NMR estimates of viscosity are widely regarded as less accurate than viscometer measurements made in fluids laboratories, NMR well logs have the following advantages: (1) Depth-continuous log data is frequently possible to obtain; (2) When it is necessary to collect data at stations, the NMR station stops are typically much shorter than fluid sampling station stops; (3) The data is available at the well site, either during the drilling process (for logging-while-drilling) or shortly afterwards (for wireline); (4) Sample integrity is not an issue since the measurements are made on oil in situ; (5) NMR well logs are very economical compared to downhole fluid sampling; and (6) NMR can measure viscosity up to at least one million centipoise. Techniques of the present invention apply, inter alia, to wireline and to logging-while-drilling NMR tools.